The present invention relates to a restraining and force applying device that is used during a drying process for lumber pieces to reduce warp, that is, reducing the amount of crook, twist, bow and cup, of the dried lumber pieces. The device applies horizontal force to clamp together the edge surfaces of the individual lumber pieces placed edge to edge in courses of lumber pieces, and maintains these clamping forces throughout the drying process and, if required, while the lumber cools. The horizontal force is parallel to the width face of the lumber, and thus to the plane of the course of lumber as opposed to vertical forces parallel to the narrower edge surfaces of lumber pieces and perpendicular to the plane of the lumber courses. A vertical force also can be applied during drying, if desired. The horizontal clamping force applied to the courses of stacked lumber pieces keeps the individual lumber pieces securely restrained and in tight edge to edge contact throughout the drying process. Under the forces applied, the lumber pieces are held straight so that the amount of warp (crook, twist, bow and cup) is significantly reduced or eliminated.
In the prior art, it has been known to vertically restrain stacks of lumber as the lumber is dried in a kiln, or by other means of drying, through the use of weights on the top of the stack, which provides a vertical downward force on the stack. The lumber is generally stacked in layers or courses with each course separated from the next overlying layer or course by spacers called “stickers”. The stickers create passageways for air movement through the stack of lumber between the courses.
The vertical load now applied on a stack of dimension lumber, such as for two-by-four studs of eight-foot length, is an attempt to reduce the warp in the individual lumber pieces as the lumber is dried. However, the effectiveness of vertical loads has been less than satisfactory, with a great deal of crook and twist of the dimension lumber occurring both during drying and after release from the lumber stack. The application of dead weight on top of the lumber stacks during drying is usually in the form of concrete blocks or a panel of steel. Also, it has been known to apply loads using hydraulic rams, again, in a vertical direction.
The most serious and degrading forms of warp in dimension lumber are crook and twist. Crook is a deviation of the narrow edges of a piece of lumber from a straight line, while twist is the rise of a corner of the piece out of a horizontal plane from one end to the other. Grading rules for each size and grade of lumber mandate specified maximum amounts of crook and twist. Straighter lumber has the potential for meeting higher grades and thus increased value. Upgrading the lumber pieces to higher grades via warp reduction produces a substantial increase in both profitability of a mill and the assurance of better performance in subsequent use of the lumber.
Presently, any resistance to crook development in individual dimension lumber pieces, such as studs, relies upon the ability of top loading to increase the frictional resistance to movement between the lumber pieces in each of the courses of lumber and the stickers used to separate the courses. The effect on reducing crook with only vertical forces is marginal, particularly for those individual lumber pieces with lower than average thickness.